Fly ash, the finely divided material collected by electrostatic precipitators from the flue gases at coal-fired power plants, has become an essential component of concrete. Concrete proportioned with fly ash shows improved performance not attainable by the use of hydraulic cement alone. Fly ash has physical and chemical synergistic effects with hydraulic cement in concrete. The type and extent of these effects depend on the particle size distribution and chemical composition of fly ash. Fly ash is classified based on its chemical composition into two categories: class F and C, per ASTM C 618. Generally, class F fly ashes have a predominant alumino-silicate glass phase which provides pozzolanic properties while class C fly ashes have a predominant calcium-alumino-silicate glass phase which is pozzolanic as well as cementitious.
The effects assumed primarily dependent of a physical interaction, such as improved workability, are the similar regardless of the type of fly ash. Conversely, the effects assumed primarily dependent on a chemical interaction such as time of setting in fresh state and temperature rise in hardened state are different depending on the type of fly ash and calcium content (CaO). This unpredictability of fly ash behavior increases as the calcium content (CaO) of the fly ash increases, which leads to higher hydraulic reactivity. Accordingly, the use high calcium fly ash in cement has traditionally been very challenging due to the high hydraulic reactivity associated with the high reactive calcium content in the high calcium fly ash.